The first low voltage, low noise differential silicon microphone, technology development and measurement results

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Abstract

The first differential silicon microphone is presented. This capacitive working device consists of two backplates with a membrane in between. Due to the balanced arrangement the air gap can be minimized. Thus, a higher electrical field and sensitivity can be achieved for low voltages. A dedicated process sequence has been developed in order to get the optimum mechanical and electrical properties for all structural layers. Furthermore, a sandwich structure has been developed to achieve a reproducible, very sensitive microphone membrane with a thickness of only 0.5 μm and a stress of 45 MPa. The total sensitivity for a bias of 1.5 V was measured to be 13 mV/Pa and the A-weighted equivalent input noise was measured to be 22.5 dB SPLA. This noise level does not correspond to the simulations where only 21.0 dB SPLA have been predicted. Modeling of the membrane using distributed resistors shows that the lumped element resistor used for the membrane resistance has been underestimated and thus, the noise level. The upper limit of the dynamic range has been determined to be 118 dB SPL and the total harmonic distortion at 80 dB SPL is below 0.26%.

Introduction

Generally, condenser microphones have the potential for a high signal-to-noise ratio [1], [2], [3]. However, silicon condenser microphones typically suffer from low sensitivity. Thus the input referred noise of the attached preamplifier stage becomes dominant which results in a high acoustical equivalent input noise. Here, a differential condenser microphone with two backplates is presented (see Fig. 1E). Due to the symmetric arrangement of the backplates, this kind of microphone ideally offers twice the signal of a single backplate microphone. Furthermore, the bias field can be 30% higher compared to single backplate microphones, because the electrostatic force acts on both sides of the membrane and therefore keeps the membrane in its zero position [4]. This also results in both a higher sensitivity and a wider linear dynamic range. In addition, a broader bandwidth can be achieved compared to the traditional hearing instrument microphones due to an acoustically stiffer system.

The process development, the sensitivity and noise measurement results will be discussed and compared with the conventional hearing instrument microphones.

Section snippets

Technology development

The microphone membrane is made of a multilayer consisting of low stress (380 MPa) silicon-rich nitride and B++ poly-Si (−40 MPa). Thus, the total stress can be adjusted using compressive and tensile stressed layers with various thicknesses. In order to find the optimum layer thickness ratio, a series of different membranes has been produced and investigated. Fig. 2 shows that it is possible to achieve a stress of about 45 MPa using 0.4 μm B++ poly-Si and a total thickness of 900 Å of silicon-rich

Measurement results

The microphones were measured on a wafer-scale using an audio analyzer, a reference microphone, a low noise amplifier, and a sound source acting from the backside of the wafer. Thus the standard probes with a low noise buffer amplifier mounted to one probe could be used. The two backplates were biased simultaneously but measured successively. The total sensitivity at 1 kHz for 2mm×2 mm microphones was measured to 13 mV/Pa and the 3-dB bandwidth to more than 20 kHz. As shown in Fig. 6, the signal

Discussion

All measurement results of the previous section were obtained by the wafer-scale measurements and therefore with an infinitely large microphone backchamber. Transferring the results to a microphone with a backchamber volume of e.g. 3.4 mm3 the performance decreases and the acoustical equivalent input noise becomes 24 dB SPLA. This is comparable with the best 20 mm3 hearing instrument microphones, but offering a much wider frequency range. The noise decreases to 23 dB SPLA if only the typical

Conclusion

This work shows that it is possible to fabricate a relatively complicated differential condenser silicon microphone with an excellent sensitivity of 13 mV/Pa and a noise level as low as 22.5 dB SPLA using only 1.5 V power supply. The THD was measured to be below 0.3 and 1.6% at a sound pressure level of 80 and 100 dB, respectively. The dynamic range is about 96 dB.

Compared to the conventional hearing instrument microphones, this kind of silicon microphone has reached the same level of acoustical

Acknowledgements

The authors thank Microelectronic Center (MIC) at the Technical University of Denmark and, especially Lis Nielsen for processing of the microphones and Manuel Mallebrera Moreno for performing the very time consuming noise measurements. This work was supported by the Danish board of Trade and Commerce and the Danish National Research Council through the Microsystems Center collaboration project (partners: MIC, DELTA, Microtronic A/S).

Pirmin Rombach received the Dipl.-Ing. degree in electronics engineering from the Technical University of Karlsruhe in 1989. The Dr.-Ing. degree in Electronics Engineering, he received from the Technical University of Darmstadt in 1995, for his work on a micromachined torque sensor at the Solid State Electronics Laboratories, where he was working as a research assistant. In 1996, he joined the research group of Microtronic in Roskilde, Denmark. The focus of his research is modeling and process

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Pirmin Rombach received the Dipl.-Ing. degree in electronics engineering from the Technical University of Karlsruhe in 1989. The Dr.-Ing. degree in Electronics Engineering, he received from the Technical University of Darmstadt in 1995, for his work on a micromachined torque sensor at the Solid State Electronics Laboratories, where he was working as a research assistant. In 1996, he joined the research group of Microtronic in Roskilde, Denmark. The focus of his research is modeling and process development for micromachined microphones and loudspeakers but also backend processing aspects for microsystems.

Matthias Müllenborn received his MSc degree in Physics from the University of Münster, Germany, in 1990 and his PhD degree in Materials Science & Engineering from the University of California at Los Angeles, USA, in 1993. His research projects focused on the optical characterization of III/V materials at the Siemens Research Labs in Munich (now Infineon) and on the investigation of the heterostructure interfaces at UCLA. From 1993 to 1996, he was employed at the Microelectronics Center of the Technical University of Denmark as a research assistant professor, working on high-resolution laser micromachining and nanoscale direct writing.

He joined Microtronic A/S, Denmark, in 1996 where he worked on the development of a microtechnology-based microphone for hearing instrument applications. He is project manager of the ESPRIT-funded project HISTACK and manager of the Microsystems Group at Microtronic.

Udo Klein received the Dipl.-Ing. and Dr.-Ing. degrees in electronics engineering from the Technical University (TU) Braunschweig, Germany, in 1986 and 1990, respectively. In 1987, he joined the Institute of High-Frequency Engineering, TU Braunschweig, working on the superconducting electronics. Between 1990 and 1993, he was a Guest Researcher at the Electrotechnical Laboratory, Japan, where he worked on Josephson voltage standards. He then worked in academic research on both low and high-temperature superconductor technology for magnetic sensor and electronic devices at the University of Birmingham, England, the University of Strathclyde, Scotland, and the Technical University of Denmark. During 1996, he visited the Department of Materials and Production Engineering, University of Naples, Italy, as a consultant for the design of a SQUID NDE system. Since he joined Microtronic A/S, Denmark, in 1997 as Microsystems Design Engineer, he has been developing microelectromechanical devices for the hearing instrument industry, particularly silicon microphones. Dr. Klein is a member of the IEEE and the IoP.

Kurt Rasmussen received the MSc in electrical engineering from the Technical University of Denmark in 1996. From 1996 he was working as a research assistant at the Microelectronics Center of the Technical University of Denmark on silicon wafer bonding. In 1999, he joined the research group of Microtronic in Roskilde, Denmark. The focus of his work is process development for micromachined microphones.

Reprinted with permission from Technical Digest of the 14th IEEE International Conference on Micro Electro Mechanical Systems, Interlaken, Switzerland, January 21–25, 2001.

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